In recent years, attention has been given to a display in which a contact detection device so-called touch panel is mounted on a display such as a liquid crystal display and various buttons are displayed on the display so that information is allowed to be inputted by the buttons instead of normal buttons. In the display including such a touch panel, an input device such as a keyboard, a mouse or a keypad is not necessary, so there is a tendency to expand the use of the displays to portable information terminals such as cellular phones in addition to computers.
On the other hand, a reduction in power consumption of various electronic devices attracts attention in response to recent interest in ecology. In the display including the touch panel, a reduction in power consumption of not only a display panel but also the touch panel is desired. In particular, the touch panel is constantly in an activation state so that a touch is detectable whenever a touch operation is performed. As a result, the power consumption of the touch panel often reaches a considerable level. Therefore, various proposals have been made as a method of reducing the power consumption of the touch panel.
For example, PTL 1 discloses a display including a liquid crystal display panel which is integrated with an optical touch panel having two operation modes (a normal operation mode and a lower power consumption mode). For example, while a user inputs information by touching the touch panel, the touch panel operates in the normal operation mode, and when a touch is not detected for a predetermined period, the touch panel shifts to the low power consumption mode. In the low power consumption mode, power consumption is reduced by reducing the frequency of touch detection operations in the touch panel. After that, when a touch is detected in the low power consumption mode, the touch panel restores its operation mode from the low power consumption to the normal operation mode, and the user is allowed to input information again by touching the touch panel.
Moreover, for example, PTL 2 discloses a display including a liquid crystal display panel which is integrated with an optical touch panel having a normal operation mode and a low power consumption mode and reducing power consumption by reducing the frequency of touch detection operations in the low power consumption mode and setting, to a sleep state, only a circuit which performs a computation of a touch position.